Systems and method for removal of acid gas in a circulating dry scrubber

ABSTRACT

Systems and methods for the use of highly reactive hydrated lime (HRH) in circulating dry scrubbers (CDS) to remove sulfur dioxide (SO 2 ) from the flue gas.

CROSS REFERENCE TO RELATED APPLICATION(S)

This application claims the benefit of U.S. Provisional Patent Application Ser. No. 62/046,696, filed Sep. 5, 2014. The entire disclosure of all the above documents is herein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to air pollution control processes aimed at controlling acid gases that are emitted from industrial, utility, incineration, or metallurgical process. Specifically the invention concerns the mitigation of Sulfur Dioxide (SO₂), Hydrochloric acid (HCl), and Sulfur Trioxide (SO₃) using a high reactivity calcium hydroxide (hydrated lime) in a circulating dry scrubber (CDS).

2. Description of the Related Art

Many efforts have been made to develop materials for improved capability of cleaning or “scrubbing” flue gas or combustion exhaust. Most of the interest in such scrubbing of flue gas is to eliminate particular compositions, specifically acid gases, that contribute to particularly detrimental known environmental effects, such as acid rain.

Flue gases are generally very complex chemical mixtures which comprise a number of different compositions in different percentages depending on the material being combusted, the type of combustion being performed, impurities present in the combustion process, and specifics of the flue design. However, the release of certain chemicals into the atmosphere which commonly appear in flue gases is undesirable, and therefore their release is generally regulated by governments and controlled by those who perform the combustion.

Some of the chemicals that are subject to regulation are certain acid gases. A large number of acid gases are desired to be, and are, under controlled emission standards in the United States and other countries. This includes compounds such as, but not limited to, hydrogen chloride (HCl), sulfur dioxide (SO₂) and sulfur trioxide (SO₃). Sulfur trioxide can evidence itself as condensable particulate in the form of sulfuric acid (H₂SO₄). Condensable particulate can also be a regulated emission.

Flue gas exhaust mitigation is generally performed by devices called “scrubbers”. Scrubbers introduce chemical compounds into the flue gas. The compounds then react with the undesirable compounds which are intended to be removed. Through these reactions, the undesirable compounds are either captured and disposed of, or turned into a less harmful compound prior to their exhaust, or both. In addition to controlling the emissions for environmental reasons, it is desirable for many combustion plant operators to remove acid gases from the plant's flue gas to prevent the acid gases from forming powerful corroding compounds which can damage flues and other equipment.

These acid gases can arise from a number of different combustion materials, but are fairly common in fossil fuel combustion (such as oil or coal) due to sulfur being present as a common contaminant in the raw fuel. Most fossil fuels contain some quantity of sulfur. During combustion, sulfur in the fossil fuel can oxidize to form sulfur oxides. A majority of these oxides forms sulfur dioxide (SO₂), but a small amount of sulfur trioxide (SO₃) can also be formed. Selective Catalyst Reduction (SCR) equipment, commonly installed for the removal of nitrogen oxides (NO_(x)), will also oxidize a portion of the SO₂ in a flue gas to SO₃.

SO₂ is a gas that contributes to acid rain and regional haze. Since the 1970's, clean air regulations have been designed to reduce emissions of SO₂ from industrial processes at great benefit to the environment and human health. For large emitters, the use of wet and dry scrubbing has led to the reduction of SO₂. Smaller emitters, however, seek out less costly capital investment to control SO₂ emissions in order to remain operating and produce electricity or steam. Similarly, halides in fossil fuels (such as chlorine and fluorine) are combusted and form their corresponding acid in the flue gas emissions. The halogenated acids also contribute to corrosion of internal equipment or, uncaptured, pollute the air via stack emissions.

However, mitigation of the above undesirable compounds can be very difficult. Because of the required throughput of a power generation facility, flue gases often move through the flue very fast and thus are present in the area of scrubbers for only a short period of time. Further, many scrubbing materials often present their own problems. Specifically, having too much of the scrubbing material could cause problems with the plant's operation from the scrubber material clogging other components or building up on moving parts.

Flue gas treatment has become a focus of electric utilities and industrial operations due to increasingly tighter air quality standards. As companies seek to comply with air quality regulations, the need arises for effective flue gas treatment options. Alkali species based on alkali or alkaline earth metals are common sorbents used to neutralize the acid components of the flue gas. The most common of these alkalis are sodium, calcium, or magnesium-based. A common method of introduction of the sorbents into the gas stream is to use dry sorbent injections. The sorbents are prepared as a fine or coarse powder and transported and stored at the use site. Dry sorbent injection systems pneumatically convey powdered sorbents to form a fine powder dispersion in the duct. The dry sorbent neutralizes SO₃/H₂SO₄, and protects equipment from corrosion while eliminating acid gas emissions. Common sorbents used are sodium (trona or sodium bicarbonate) or calcium (hydrated lime or Ca(OH)₂) based.

One commonly used material for the scrubbing of acid gases is hydrated lime. It has been established that hydrated lime can provide a desirable reaction to act as a mitigation agent. Hydrated lime reacts with SO₃ to form calcium sulfate in accordance with the following equation: SO₃(g)+Ca(OH)₂(s)→CaSO₄(s)+H₂O(g)

Hydrated lime systems have been proven successful in many full scale operations. These systems operate continuously to provide utility companies with a dependable, cost-effective means of acid gas control.

These hydrated lime compositions specifically focus on high surface area based on the theories of Stephen Brunauer, Paul Hugh Emmett, and Edward Teller (commonly called the BET theory and discussed in S. Brunauer, P. H. Emmett and E. Teller, J. Am. Chem. Soc., 1938, 60, 309, the entire disclosure of which is herein incorporated by reference). This methodology particularly focuses on the available surface area of a solid for absorbing gases—recognizing that a surface, in such circumstances, can be increased by the presence of pores and related structures. The most effective hydrated lime sorbents for dry sorbent injection have high (greater than 20 m²/g) BET surface area.

Two examples of such compositions with increased BET surface areas are described in U.S. Pat. Nos. 5,492,685 and 7,744,678, the entire disclosures of which are herein incorporated by reference. Because of this, commercially available products are currently focused on obtaining lime hydrate with particularly high BET surface areas. It is generally believed that the BET surface area needs to be above 20 m²/g to be effective and, in many recent hydrated lime compositions, the BET surface area is above 30 m²/g in an attempt to continue to improve efficiency. These sorbents offer good conveying characteristics and good dispersion in the flue gas, which is necessary for high removal rates. Use of a higher quality, high reactivity source of hydrated lime allows for better stoichiometric ratios than previous attempts that utilized lower quality hydrated lime originally targeted for other industries such as wastewater treatment, construction, asphalt, and the like.

The reaction of hydrated lime with acid gas (such as SO₃) is generally assumed to follow the diffusion mechanism. The acid gas removal is the diffusion of SO₃ from the bulk gas to the sorbent particles. Thus, high surface area does not itself warrant a prediction in improved removals of acid gases. Specifically, high pore volume of large pores is generally believed to be required to minimize the pore plugging effect and therefore BET surface area has been determined to be a reasonable proxy for effectiveness of lime hydrates in removal of acid gases. Conventional wisdom also indicates that smaller particles act as better sorbents.

Lime hydrate meeting the above described characteristics, properties, and reactivity has generally been manufactured according to a commonly known and utilized process. First, a lime feed of primarily calcium oxide (commonly known as quicklime) is continuously grinded using a pulverizing mill until a certain percentage of all the ground particles meet a desired size (e.g., 95% or smaller than 100 mesh). In other words, all of the lime feed is ground together (lime and impurities), without any removal of particles during the grinding, until the batch of lime feed (both the lime and impurities) meets the desired particle size requirements. This continuous grinding is not surprising as the conventional wisdom is that small particles are better and, thus, the more the calcium oxide is grinded, the better.

Second, the quicklime meeting the desired size requirements is then fed into a hydrator, where the calcium oxide reacts with water (also known as slaking), and then flash dried to form calcium hydroxide in accordance with the following equation: CaO+H₂O→Ca(OH)₂

Finally, the resultant calcium hydroxide (also known as hydrated lime) is then milled and classified until it meets a desired level of fineness and BET surface area.

SUMMARY OF THE INVENTION

The following is a summary of the invention, which should provide to the reader a basic understanding of some aspects of the invention. This summary is not intended to identify critical elements of the invention or in any way to delineate the scope of the invention. The sole purpose of this summary is to present in simplified text some aspects of the invention as a prelude to the more detailed description presented below.

There are described herein systems and methods for the use of highly reactive hydrated lime (HRH) in circulating dry scrubbers (CDS) to remove sulfur dioxide (SO₂) from the flue gas.

In an embodiment, there is described herein, a system for removal of sulfur dioxide (SO₂) from a flue gas, the system comprising: a flue gas duct including a circulating dry scrubber (CDS); and an injection system for injecting a highly reactive lime hydrate (HRH) into the flue gas in the CDS.

There is also described herein, in an embodiment, a method for removal of sulfur dioxide (SO₂) from a flue gas, the method comprising: providing a flue gas duct including a circulating dry scrubber (CDS); and injecting a highly reactive lime hydrate (HRH) into the flue gas in the CDS.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 provides a conceptual block diagram of an embodiment of a flue gas duct system as may be used is, for example, in a coal fired power plant including common components.

FIG. 2 provides a conceptual block diagram of a circulating dry scrubber (CDS).

FIG. 3 provides a table illustrating increased reduction of SO₂ with a high reactivity lime hydrate (HRH) compared to a more traditional hydrated lime composition in a circulating dry scrubber to control stack SO₂ emissions.

FIG. 4 provides a graph showing average feed rates of HRH and traditional lime hydrate at a full load.

FIG. 5 provides a bar chart showing relative feed rates of HRH and traditional lime hydrate at different load conditions.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Most flue gas scrubbing systems are commonly focused on making sure that certain materials do not leave the flue stack and disperse in the air for environmental reasons. FIG. 1 shows a loose block diagram of an arrangement of a flue gas duct system such as can be used in a coal fired power plant. Generally, major components include the boiler (101), a selected catalytic reduction (SCR) system for reducing NO), emissions (103), an air preheater (APH) (105), a bag house or electrostatic precipitator (ESP) (107), a Flue Gas Desulfurization (FGD) unit (109), and then the exhaust stack (111).

Traditionally the FGD system (109) has utilized wet flue gas desulfurization (WFGD) which provides wet lime or limestone scrubbers where the calcium source (lime or limestone) is mixed in a slurry and introduced to the gas stream in a large reactor to “scrub” the SO₂ from the gas stream. Most coals burned in these plants contain small concentrations of chlorine. The chlorine is readily scrubbed from the gas within the WFGD system. As the calcium slurry is recirculated in the scrubber, these chlorine compounds that have been removed (“chlorides”) accumulate and, if not controlled, will concentrate in the slurry. As chloride concentrations rise, they begin to cause operational issues with the WFGD system due to their corrosive nature which increases with concentration. If left uncontrolled, these chlorides can attack even the most exotic and expensive metal alloys causing major damage to an WFGD system. In order to control this chloride corrosion phenomenon, it is common practice to control the level of chlorides present in the scrubber slurry by removing or “purging” a portion of the scrubber slurry. This process is often referred to as a “chloride purge”.

While WFGD is a very effective technology for scrubbing SO2, The WFGD purge stream is an aqueous solution and generally contains a wide variety of pollutants making it a rather toxic material to handle and dispose of. It, includes gypsum, along with heavy metals, chlorides, magnesium and dissolved organics. In many applications, WFGD purge water is first treated by dewatering to separate synthetic gypsum cake which can be a valuable secondary product. The remaining WFGD purge water is then recycled back to the scrubber. A portion of this water (still containing dissolved chlorides) is removed from the recycle stream (the “chloride purge”), and is subjected to various forms of water treatment (as required) to reduce or eliminate dissolved metals and other contaminants of concern prior to discharge back into the environment in accordance with the applicable permits and laws. As should be apparent, this process is both resource intensive and as regulations on allowed discharge tighten, increasingly difficult to use.

Recently, there has been an interest in the use of circulating dry scrubbing (CDS) technology. A CDS is also referred to as circulating fluid bed (CFB). The technology of a CDS is relatively straightforward and is illustrated in FIG. 2. Essentially flue gas (200) is directed into the reaction vessel (201) where hydrated lime (203) (possibly with additional water (205)) is directed into the flue gas stream. This causes the hydrated lime to react with the SO₂ to produce calcium sulfite according to Equation 1. Ca(OH)₂+SO₂→CaSO₃+H₂O  Equation 1 The calcium sulfite may further react with available oxygen to produce calcium sulfate (CaSO₄).

Once the reaction is completed. The sulfates, sulfites, and fly ash in the flue gas stream are filtered out using a fabric or electrostatic filter (207) (which often is the ESP or BH 107) allowing for clean flue gas (209) to be discharged to the stack (111). The remaining material, including the bulk of the fly ash and remaining hydrate lime, are returned (211) to the inlet of the reaction vessel (201) to provide for reuse while the bulk of the reaction products are removed and disposed of (213). Thus, hydrated lime is rarely wasted as unreacted sorbent is cycled back into the process for reuse. However, hydrated lime is also available to react with other components of the flue gas, the majority of which is carbon dioxide. This reaction, which foul's calcium carbonate (CaCO₃), reduces the efficiency of the hydrated lime both initially and as it is being recirculated and necessitates use of a molar excess of hydratated lime for sufficient reaction with SO₂.

With the development of a highly reactive lime hydrate (HRH) with properties designed to significantly improve the speed of reaction with acid gases present in flue gas, it became a possibility that HRH may be useable in CDS systems. HRH can be manufactured in accordance with a number of processes, but is contemplated herein as material manufactured in accordance with, and having the properties discussed in, U.S. patent application Ser. Nos. 14/180,128 and 14/289,278, the entire disclosure of both of which is herein incorporated by reference. HRH is notably different from other hydrated lime as it has an improved removal rate of acidic pollutants present in the flue gas where the sorbent is delivered and the rate of removal is generally substantially higher. The use of a high purity, highly reactive hydrated lime such as HRH will have faster neutralization of acidic species.

While it may be provided in a variety of forms, in an embodiment, the HRH is a dry solid free of excess moisture. The product used may be described by having citric acid reactivity of less than 15 sec, preferentially less than 10 sec, optimally less than 7 sec. The product used may also be described by having available calcium hydroxide concentration of greater than 92% wt, preferentially greater than 94%, and optimally greater than 95%. The product used may also be described by having at least 90% of particles less than 10 microns, preferentially less than 8 microns. At least 50% of the particles are less than 4 microns, preferentially less than 3 microns. The product may also be described as having a BET surface area of at least 18 m²/g, preferentially at least 19 m²/g, 20 m²/g, or 30 m²/g.

The HRH will generally be used as part of circulating dry scrubber system of the off gas of an industrial plant, incinerator, or boiler that combusts sulfur and/or halogenated fuels. Hydrated lime (203) is fed from a silo into a conveying line that disperses the fine powder into the bottom of a reactor (201) in the off gas piping. A predetermined amount of recycled ash (211) is also fed into the reactor (201), as is a quantity of spray water (205) designed to wet the solid particles and drop flue gas temperature. At the top of the reactor (201), the flue gas travels through the duct into a BH (207)/(107), where ash collects on the bags while clean flue gas (209) flows through the ash/bag layers. Automated mechanical means dislodge the ash from the bag exterior and this ash is either recycled (211) to the reactor (201) or sent (213) to a landfill or other beneficial use as deemed appropriate.

Because of the highly reactive nature of HRH, in a CDS a finer cloud from the sorbent injection lance puts more sorbent particles in the pathway of acid gases, reaching stratified areas and neutralizing more acid components. The use of a high purity, highly reactive hydrated lime such as HRH will have faster neutralization of acidic species which can be problematic when present in flue gas. Such reactivity enhancement is beneficial especially when the finer cloud from the sorbent injection is used as an input into the scrubber reactor portion of a CDS. This puts more sorbent particles in the pathway of acid gases, reaching stratified areas and neutralizing more acid components.

While CDS systems are perceived to be quite efficient due to multiple cycles of sorbent through the reactor section (That the lime sorbent is recirculated (211)), a majority of acid gas pollutant reduction occurs the first cycle through the system (reactor (201) to BH (207)/(107) in a common CDS process). A sorbent with improved capability for in-flight capture exhibits better removal in the reactor (201) than standard sorbent, thus providing better pollutant control (typically monitored via SO₂ emission monitoring) with lesser quantities of sorbent.

In an exemplary embodiment, a plant fueled with PRB coal uses a CDS to control stack SO₂ emissions. In this embodiment, the plant compared a traditional hydrated lime against performance of an HRH. The CDS operates under a logic-based controller that adjusts the lime feed rate in order to maintain a near constant SO₂ emission as determined by a CEMS monitor in the stack flue gas. The unit and scrubber were operated normally over several days with each type of hydrated lime. Results are outlined in FIG. 3.

As shown in FIG. 3, at all load ranges the CDS required significantly less HRH than traditional hydrated lime in order to maintain desired SO₂ emission limits. Scrubber inlet and outlet temperatures are provided to show that temperature variations in the scrubber were not the reason for improved performance with HRH. The experiment was conducted with the CDS first operating on the traditional hydrated lime, then converted to HRH, then converted back. FIG. 4 shows how the plant operated utilizing substantially less sorbent in the HRH window than in either of the others. Further, FIG. 5, which was obtained from a longer term test than that of FIG. 3 shows that the HRH showed dramatic reductions in use at lower loads. Specifically while the reduction at high loads (specifically those of 270 MW or greater in the selected test facility) was good, there is a dramatic reduction at medium loads (between 200-270 MW) and the drop is even more pronounced at low loads (between 125-250 MW). Thus, HRH has shown surprising benefit particularly in low load conditions.

The development of a high purity, fast reacting hydrated lime with a narrow, small particle size distribution improves dispersion in the flue gas when the sorbent is delivered to the process. These properties provide a sorbent that increases coverage of the pathway of acid gases and rapidly reacting with those gases to neutralize acidic species in the flue gas.

Without being limited to any theory of operation, increased acid gas capture in-flight improves the sorbent efficiency the first time through the reactor (201) and BH (207)/(107). Since fresh sorbent is believed to capture the most pollutant, there should be differentiation in performance between a traditional hydrated lime and an HRH, but the performance is better than expected. This differentiation evidences itself via reduced hydrated lime requirements to the CDS.

In an embodiment, the use of HRH provides a method of removing SO_(x) from flue gas of boiler firing sulfur-containing fuel that is advantageous over prior art due to tighter particle size distribution of the hydrated lime sorbent. Advantageous generally refers to reduced sorbent quantities required to achieve similar reduction in concentration of SO_(x) that end user requires for traditional hydrated lime.

In an embodiment, this also provides a method of removing SO₂ from flue gas of boiler firing sulfur containing fuel that is generally advantageous due to more rapid reactivity of hydrated lime sorbent, as characterized by acid reactivity test. Here, advantageous refers to greater reductions in concentration of SO₂ that end user experiences when using standard hydrated lime.

While the invention has been disclosed in conjunction with a description of certain embodiments, including those that are currently believed to be the preferred embodiments, the detailed description is intended to be illustrative and should not be understood to limit the scope of the present disclosure. As would be understood by one of ordinary skill in the art, embodiments other than those described in detail herein are encompassed by the present invention. Modifications and variations of the described embodiments may be made without departing from the spirit and scope of the invention.

It will further be understood that any of the ranges, values, properties, or characteristics given for any single component of the present disclosure can be used interchangeably with any ranges, values, properties, or characteristics given for any of the other components of the disclosure, where compatible, to form an embodiment having defined values for each of the components, as given herein throughout. Further, ranges provided for a genus or a category can also be applied to species within the genus or members of the category unless otherwise noted. 

The invention claimed is:
 1. A system for removal of sulfur dioxide (SO2) from a flue gas, the system comprising: a flue gas duct including a circulating dry scrubber (CDS); and an injection system for injecting a highly reactive lime hydrate (HRH) into said flue gas in said CDS.
 2. A method for removal of sulfur dioxide (SO2) from a flue gas, the method comprising: providing a flue gas duct including a circulating dry scrubber (CDS); and injecting a highly reactive lime hydrate (HRH) into said flue gas in said CDS. 